Raman spectroscopy is a technique for performing chemical analysis. High intensity monochromatic light, such as that provided by a laser, is directed onto an analyte molecule (or sample) that is to be chemically analyzed. A majority of the incident photons are elastically scattered by the analyte molecule, the elastically scattered photons having the same energy (and, therefore, the same frequency) as the incident photons. This elastic scattering is termed Rayleigh scattering, and the elastically scattered photons and radiation are termed Rayleigh photons and Rayleigh radiation, respectively. However, a small fraction of the photons (e.g., about 1 in 107 photons) are inelastically scattered by the analyte molecules. These inelastically scattered photons have a different frequency than the incident photons. This inelastic scattering of photons is termed the Raman effect. The inelastically scattered photons may have frequencies greater than, or, more typically, less than the frequency of the incident photons.
When an incident photon collides with a molecule, energy may be transferred from the photon to the molecule or from the molecule to the photon. When energy is transferred from the photon to the molecule, the scattered photon will emerge from the sample having a lower energy and a corresponding lower frequency. These lower-energy Raman scattered photons are commonly referred to in Raman spectroscopy as the Stokes radiation. A small fraction of the analyte molecules are already in an energetically excited state. When an incident photon collides with an excited molecule, energy may be transferred from the molecule to the photon, which will emerge from the sample having a higher energy and a corresponding higher frequency. These higher-energy Raman scattered photons are commonly referred to in Raman spectroscopy as the anti-Stokes radiation.
The Stokes and the anti-Stokes radiation is detected by a detector, such as a photomultiplier or a wavelength-dispersive spectrometer, which converts the energy of the impinging photons into an electrical signal. The characteristics of the electrical signal are at least partially a function of the energy (or wavelength, frequency, wave number, etc.) of the impinging photons and the number of the impinging photons per unit time (intensity). The electrical signal generated by the detector can be used to produce a spectral graph of intensity as a function of frequency for the detected Raman signal (i.e., the Stokes and anti-Stokes radiation). A unique Raman spectrum corresponding to the particular analyte may be obtained by plotting the intensity of the inelastically scattered Raman photons against their frequency or, equivalently and more commonly, their wavenumber in units of inverse centimeters. A Raman spectrum readout is often presented as a plot of intensity versus Raman shift, the Raman shift being defined as a difference between the wavenumbers of the source radiation (excitation radiation) and the Raman-scattered radiation. Peaks and valleys that are meaningful for purposes of chemical analysis are typically for Raman shifts in the range of 500 cm−1-2000 cm−1, which for a typical source wavelength of 1000 nm would correspond to Raman-scattered photons having wavelengths between 1050 nm-1250 nm.
This unique Raman spectrum may be used for many purposes such as identifying an analyte, identifying chemical states or bonding of atoms and molecules in the analyte, and determining physical and chemical properties of the analyte. Raman spectroscopy may be used to analyze a single molecular species or mixtures of different molecular species. Furthermore, Raman spectroscopy may be performed on a number of different types of molecular configurations, such as organic and inorganic molecules in either crystalline or amorphous states.
Unfortunately, molecular Raman scattering is a weak “two photon” process of lower probability than “single photon” processes such as IR absorption, and is thus a more difficult process to measure. Powerful, costly laser sources typically are required to generate high intensity excitation radiation to increase the weak Raman signal for detection. Surface enhanced Raman spectroscopy (SERS) is a technique that allows for generation of a stronger Raman signal from an analyte relative to non-SERS Raman spectroscopy for a sample with the same number of analyte molecules. In SERS, the analyte molecules are adsorbed onto, or placed adjacent to, an activated metal surface or structure, termed herein a SERS-active structure. The interactions between the molecules and the surface cause an increase in the strength of the Raman signal. Several SERS-active structures have been employed in SERS techniques, including activated electrodes in electrolytic cells, activated metal colloid solutions, and activated metal substrates such as a roughened metal surface or metal islands formed on a substrate.
Hyper Raman spectroscopy refers to the analysis of Raman signals near a second (or higher) harmonic of the monochromatic excitation beam. Hyper Raman signals, also termed harmonic Raman signals, result from non-linear effects in which the vibrational modes of the analyte molecule(s) interact with the second (or higher) harmonic of the excitation beam. Hyper Raman spectroscopy can provide valuable additional insight into the characteristics of the analyte molecules, but requires very high power to achieve appreciable spectral readings. SERS-type Raman signal enhancement approaches can often be used to boost the hyper Raman spectrum as well.
Issues arise and/or remain with respect to one or more of the above-described Raman signal enhancement methods that are at least partially resolved by one or more of the embodiments described herein. For example, the Raman signal intensification produced by a SERS-active material can be highly sensitive to small variations in the localized positions of structures, bumps, or cavities therein. Substantial portions of a particular SERS-active surface can be non-enhancing, while only a small number of unpredictably located “hot spots” on that surface are effectively enhancing. It would be desirable to provide for Raman signal intensification having one or more of increased Raman signal enhancement, increased spatial uniformity of response across the sample, and increased range of analytes that can be spectroscopically analyzed. As another example, it would be desirable to provide for increased intensification of second (or higher) harmonic Raman signals. Other issues arise as would be apparent to one skilled in the art upon reading the present disclosure.